1. Field of the Invention
The present invention relates to a surface-potential distribution measuring apparatus, an image carrier, and an image forming apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatuses such as a laser printer and a digital copier focus light beams from a light source modulated according to image information on a photoconductor via a scanning optical system, and scans the light beams on the photoconductor in a predetermined direction (main scanning direction), thereby forming an electrostatic latent image on the photoconductor. The image forming apparatus makes toner adhere to the electrostatic latent image to obtain a toner image, and transfers the toner image onto a recording medium to form an output image.
The electrostatic latent image formed on the photoconductor affects the quality of the output image. Therefore, various technologies for evaluating the electrostatic latent image formed on the photoconductor are proposed. A result of the evaluation is fed back to the design of the image forming apparatus to improve the quality of the output image.
For example, Japanese Patent Application Laid-open No. H3-49143 has proposed a conventional technology of scanning a sample surface with an electron beam, and observing an electrostatic latent image using a secondary electron emitted by the scanning. The conventional technology requires time to prepare the observation. Therefore, when the sample is a normal dielectric substance that can permanently hold the charge, this sample can be observed. However, when the normal photoconductor is used as a sample, this sample cannot hold the charge during a long time, and, for example, the electrostatic latent image disappears at the observation preparation stage, making it impossible to accurately observe the sample.
The present applicant has proposed a method of measuring an electrostatic latent image that can be observed even when a photoconductor for use cannot hold the charge during a long time. Reference may be had to, for example, Japanese Patent Application Laid-open Nos. 2003-295696, 2003-305881, 2004-251800, and 2004-233261.
In recent years, along the rapid progress of digitalization of image information, demand for higher quality of the output image of the image forming apparatus is increasing. To meet this demand, further improvement in the precision of evaluation of the electrostatic latent image is desired.
A vibrating reed electrometer has been available as a commercial product can measure a potential of a few hundred to a few thousand volts in a potential resolution of a few volts. However, because a sensor probe needs to be set to a position with a distance from the sample in principle, space resolution is only a few millimeters or above, and the vibrating reed electrometer finds it difficult to measure a potential in a region of 1 millimeter or below.
Japanese Patent Laid-open No. H3-49143 discloses a conventional technology of observing the electrostatic latent image using an electron beam. According to this conventional technology, a sample to be observed is limited to a large-scale integration (LSI) chip and a sample that can store and hold the electrostatic latent image. That is, the normal photoconductor that generates a dark attenuation as described above cannot be measured. Because the normal dielectric substance can semipermanently hold a charge, even when the potential is measured taking time after forming a charge distribution, a result of the measurement is not affected. However, because a resistance value of the photoconductor is not infinitely large, the photoconductor cannot hold the charge during a long time. As a result, a dark attenuation occurs, and the surface potential decreases along lapse of time. The photoconductor can hold a charge for about a few dozens of seconds at most in the dark room. Therefore, even when an electrostatic latent image is attempted to be observed with a scanning electron microscope (SEM) after charging and exposure, the electrostatic latent image disappears during the preparation stage.
As disclosed in Japanese Patent Application Laid-open No. 2003-295696, the present applicant has proposed a system that measures an electrostatic latent image even when a photoconductor sample has dark attenuation. That is, when a sample surface has a charge distribution, an electric field distribution corresponding to the surface charge distribution is formed in space. Therefore, this electric field pushes back a secondary electron generated by an incident electron (charged particle beam), and decreases the amount of beam reaching the detector. Accordingly, a part having strong electric field intensity has a dark contrast and a part having weak electric field intensity has a bright contrast, enabling a contrast image to be detected corresponding to the surface charge distribution.
Therefore, when the photoconductor sample is exposed with light, an exposed part turns black and a non-exposed part turns white, thereby forming an electrostatic latent image that can be measured.
Japanese Patent Application Laid-open Nos. 2003-305881, 2005-166542, and 2005-221935 also discloses conventional technologies for measuring an electrostatic latent image or surface-potential distribution using a charged particle beam.
However, with these conventional technologies, it has been difficult to measure a surface electrification and a surface potential in high resolution of a micron order, and there has been room for improvement in evaluating the electrostatic latent image formed on the photoconductor of the image forming apparatus.